Transformer



June 8 1926. 1,588,162

B. BRADBURY TRANSFORMER Filed March 27, 1922 InVehtor: A Burke BTadbL+Tf Maw His Atfiolney,

Patented June 8, 1926 "UNITED STATES 1,588,162 .PATENT OFFICE.

BURKE BRAIDBUBY, OF SCHENECTADY, NEW YORK, ASSIGNOB TO GENERAL ELECTRIC COMPANY, A GORPORATION 0F 31 EW YORK.

TRANSFORMER.

Application filed March 27, 1922. Serial No. 547,001.

' characteristic of my invention are set forth I desired.

with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation will best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is an elevation of one of my devices; Fig. 2 is a vertical cross-section; Fig. 3 is an elevation of one-half of a transformer showing the interior construction; and Fig. 4 shows a circuit arrangement in which my transformer may be employed.

As shown in the drawing, the transformer comprises a pair of similar coils 1 and 2 separated by a thin sheet 3 of insulating material? The coils 1 and 2 surround an open core & inade up of thinlaminations of enameled iron preferably having a thickness of one and one-half mils or less. .The coils are enclosed in a casing of insulating material made upof two similar parts 5 and 6 which may be fastened together by means of the screw bolts 7 and 8 which also may serve for securing to the casing the supporting legs 9 and 10. Each coil is provided with end terminals 11 and 12, and an intermediate terminal 13. Switches 1 1 and 15 which are mounted on the casing permit of the short circuiting of the portions of the coils between terminals 12 and 13.

In thev circuit arrangement shown in Fig. 4, high frequency currents to be amplified are impressed from the antenna 16 upon the input circuit of athree-electrode vacuum tube amplifier 17. The coil 1 is included in the output circuit of amplifier 17 and the coil 2 is included in the input circuit of a second amplifier 18. As many amplifier stages as desired may be connected'up in this way and the transformer in the output circuit of the last amplifier of the series may be connected to a detector of any form In the construction of radio frequency characteristics.

transformers in the past the usual practice has been to reduce to a minimum practical value the capacity between turns of the winding. This has resulted in transformers which were eiiicient over a very narrow which dropped off rapidly on either side of this range. I have found however that by making the capacity between turns as high as practical a transformer will be obtained which is efiicient over a much wider range of wave lengths and one, the efficiency of which does not fall off so rapidly over either side of the most efficient range. In order to give the coils as high capacity as possible I employ as thin insulation as possible to secure the necessary insulation between turns, and arrange the winding in compact form. The capacity may be further increased by impregnating the coil with insulating material such as a waxhaving a high dielectric constant. 0

60 range of wave lengths and the efiiciency of l. have also found that the use of a core of thin magnetic material such as I have described, greatly increases the range of wave lengths over which the transformer operates efliciently. For short wave lengths over a range which may extend from 200 to 1500 meters the magnetic skin effect is so great that the iron has practically no effect upon the operation of the device. At longer wave lengths the magnetic skin efi'ect decreases and the iron becomes more effective causing a gradual increase in the inductance of the coils as the wave length increases. This results in broadening the tuning of the coils and hence renders the transformer efficient over a greater range of wave lengths than one which has coils with sharp tuning In the operation of my transformers over a range of wave lengths from 200 to 500 meters the switches 14 and 15 are closed so that the sections of both coils connected thereto are short circuited while over the range from 500 to 5000 meters the switches 14. and 15 are opened so that the full windings of both primary and secondary coils are used. This short circuiting is not detrimental to the'operation of the transformer, since the number of turns short circuited is such that their impedance is very high for the frequencies used with this condition. On the contrary this seems to be an advantage as I have found that it permits of the use i at hundred to five thousand or above the efficlency 1s higher than thatof a similar trans f former without an iron core. Below about of a coil of a greater number of turns than -would ordinarily be employed for short waves and it is only by the use of such a coil that the desired broadening of the tunturns and produce a high impedance circuit.

ll construct my transformers with a one to one ratio because of the fact that with ltd dill

dill the usual amplifying tubes the internal .capacity between elements is so great that the impedance of the input circuit, of one tube is of. about the same order of magnitude as the impedance of the output circuit of the preceding tube, Furthermore, it appears that for short wave lengths the natural period of the coils should be in the neighborhood of the frequencies used. llf a step-up ratio were employed the primary and secondary windings would have difi'erent natural periods and this would 'result' in less transfer of energy from one tube to the next.

To enable one to successfully practice my invention the following, example is given of the physical characteristics of a transformer which l have used extensively" in carrying my invention into edect. The two coils are. each wound with about 4:65 turns of five mil single silk insulated and enameled wire."

The total resistance of each coil is about 70 ohms and the resistance of the portion used for short waves is about 20 ohms. llhe internal diameter of the coils is about and external diameter about 1 and the coils are about 3 g long. The ,core is 1 long and has a cross section of about sq. in.

A transformer constructed asdl have described has an efficiency of transformation as determined by audibility amplification measurements over a range of wave lengths from two hundred to about fifteen hundred meters at least as high as that of a similar transformer without the iron core while over a range of wave lengths from about fifteen neearea five hundred meters the transformer operates at highest efiiciency with the switches 14. and 15 closed while above that wave length it operates with higher eficiency with these switches open.

What ll claim as new and desire to secure by Letters Patent of the United States, 1s

1. A radio frequency transformer comprising a pair of coils having similar characteristics in close proximity to each other, said coils having appreciable capacity between turns, an open core for said coils made up of thin laminations of magnetic material, a casing for said coils and switches mounted on such casing, said switches being connected to suitable taps on said coils for short circuiting such portion of the turns thereof that the impedance of the portion of the remaining portion.

2. A radio frequency transformer comprising a pair of coils having similar char acteristics, said coils having appreciable capacity between turns, an open core for said coils made up of thin laminations of magnetic material and means for short circuiting such portions of each of said coils that the impedance of the portions to be short circuited will be greater than that of the remaining portions. 3, A radio frequency transformer comprising a pairof coils having similar characterstics; said coils having appreciable "capacity between turns, an open core for said coils made up of thin laminations of magnetic material and means for short circuiting such portions ofeach of said coils that the impedance of the portions to be short circuited will be greater than that of the remaining portions, the eficiency of transformation of said transformer below a wave length of about five hundred meters being greater with such portions of the coils short Gill "to be short circuited will be greater than that til lllll lot lltl 

